1. Technical Field
This invention relates to a method and an apparatus for measuring the volume of flow of a fluid through a conduit, especially the flow of blood from the heart.
2. Description of Related Art
The ability to measure the volume of fluid flow in conduits is important in many different types of applications. Such applications range from measuring the amount of oil that flows through a pipeline to measuring the volume of blood that the heart is pumping.
The measure of volumetric flow of blood from the heart presents particular problems. First, the flow of blood in the vascular system is generally non-uniform. Second, the measurement device used should obviously not be more intrusive than necessary, not only to avoid reducing the accuracy of measurements but also to avoid interfering with the normal operation of the heart; otherwise, the measurement process itself may be more dangerous for a patient than any condition the measurement system is intended to discover. Third, the accuracy of measurement systems for cardiovascular flow suffers from the presence of often pronounced disturbances such as the periodic, pulsating nature of the flow and other frequency-related disturbances such as are caused by the breathing of the patient.
Because cardiac output is often a very important diagnostic indicator, there are a large number of devices for measuring blood flow in the vascular system. In many passive measurement systems, some irregular indicator such as variations in optical translucence or magnetic irregularities are observed at two points in the blood vessel. Using auto- and cross-correlation techniques, blood flow is estimated as PG,3 a function of blood velocity, which is in turn derived as a product of the correlation technique.
In active measurement systems, the indicator is injected into or is applied to the bloodstream, whereupon blood flow is determined as in passive systems either by direct measurement of the time it takes for some quantity of the indicator to pass between two measurement points, or by using some correlation technique. The indicators used in such systems include actual substances such as dyes and radioactive particles, and pure-energy indicators such as ultrasound and heat.
U.S. Pat. No. 4,507,974 (Yelderman, 2 Apr., 1985), and U.S. Pat. No. 4,236,527 (Newbower et al., 2 Dec., 1980), describe systems for measuring cardiac output in which heat is used as an indicator. In such heat-based systems, a balloon catheter is typically threaded down through the right jugular vein, and lodges proximal to the branch of the pulmonary artery via the right atrium and the right ventricle. The catheter includes a resistive heating element, which is positioned in the atrium and/or ventricle, and a thermistor, which is positioned in the artery.
In the Newbower system, the heating element is energized in such a way that the thermal energy applied to the surrounding blood has at least two frequency components, either a fundamental and one or more harmonics, or as a square-wave signal, which can also be resolved into a fundamental frequency and a number of harmonics. The temperature of the blood downstream is then measured by the thermistor and the corresponding electrical signal is filtered with respect to the fundamental frequency and at least one other frequency. Cardiac output is then estimated based on an approximate reconstruction of the transfer function of the local vascular system.
The Yelderman system energizes the heater according to a pseudo-random sequence of square waves that are derived based on a binary maximum length sequence. Correlation techniques are then used to extract from the thermistor signal an estimate of the flow rate of blood from the heating element to the thermistor.
Because the thermal noise in a vascular system is typically great, especially in and near the heart, the problem of a low signal-to-noise ratio reduces the efficiency of many heat-based measurement systems such as the Newbower and Yelderman systems. In other words, the information-carrying heat signal may, to a greater or lesser extent, be "drowned out" by the variations in temperature produced by the vascular system itself.
A seemingly obvious way to increase the signal-to-noise ratio and improve the efficiency of the measurement system would be simply to increase the power of the signal itself. In the context of heat-based systems for measuring cardiac output, this means increasing the heat generated by the heating element. This approach is, however, often impractical or impossible in systems for measuring cardiac output, since tissue or blood damage could result if the local blood temperature rises too far above normal; for example, temperatures above 50.degree. C. would almost always cause some damage.
A second problem that affects frequency-based detection systems is that there are strong natural frequency components of the body itself in the frequency range in which heat-based systems typically operate. For example, if the patient's ventilation frequency (either natural or mechanically induced) is 0.2 Hz and the excitation frequency of the heating element is also 0.2 Hz, the downstream filtering and correlation system may not be able to distinguish between the two sources and the estimate of blood flow may become unreliable.
One way to counteract this problem is to include several different frequency components in the heat signal injected into the blood. Using the Newbower system, for example, one preferably selects the fundamental frequency of the injected heat signal such that it is located at a noise minimum in the noise profile of the cardiac system. One drawback of such an approach is that one must know what frequency range contains the noise minimum in order to tune the system. Furthermore, it is difficult or impractical to modulate blood temperature at frequencies well above any significant "noisy" frequency range while keeping the system within the power and size limitations dictated by its use in the heart.
The pseudo-random square-wave heat signals used in the Yelderman system alleviate some of the problems of frequency selection in a "non-noisy" range by generating the heat signal itself to have several frequency components of approximately the same amplitude with approximately equal spacing within a frequency band. This increases the likelihood that at least some of the frequency components are in a "non-noisy" range. Moreover, the correlation techniques used in the Yelderman system typically will reject noise better than the conventional filters used in the Newbower system.
One shortcoming of the pseudo-random technique used by Yelderman is that the average power of the signal applied to the blood is only approximately half the peak power, that is, the pseudo-random signal has a duty-cycle of approximately fifty per cent. An additional weakness of the pseudo-random technique is that the number of fundamental frequency components generated is no greater than the number of steps in the maximum length sequence used. For example, assume that the pseudo-random generator generates a sequence of length 15 with a period of 10 seconds. At most 15 fundamental frequency components can then be generated in the frequency range of 0.1 Hz-10 Hz.
The pseudo-random excitation signal is an approximation to a signal that has a continuous and flat spectrum within a given frequency range. A flat spectrum, or at least a large number of significant frequency components in a given frequency range, is desirable since the more frequency components a signal has that do not correspond to a frequency in the noise spectrum, the easier it will generally be to detect the signal in the presence of the noise.
Yet another drawback of systems that assume the use of a square-wave heat signal is that it requires a relatively high amount of power to cause a heating element's temperature to rise and fall sharply enough to approximate the rising and falling edges of the square-wave. Even if one were to implement such a heating element, the thermal properties of the blood, which must be taken as they are, make it even more difficult to realize the intended-square wave signal shape.
Problems similar to those just described with respect to measuring cardiac output are also encountered measuring the flow of fluids other than blood. What is needed is therefore a system and a method for measuring fluid flow that has a lower peak-to-average power ratio than systems such as the known pseudo-random excitation system, that do not require rapid temperature changes in the fluid, that are relatively easy to implement, that effectively avoid disturbance frequencies, and that can realize a relatively high signal-to-noise ratio. The system and method should ideally be suitable for use within the vascular system to measure cardiac output. It is the object of this invention to provide a system and a method that meet these goals.